This invention relates to air filtration devices and specifically to those whose objective is to remove certain gaseous components from an air flow. It is well-known that, in any space having a recycled air system, it is highly desirable to filter the recycled air to remove particulate matter. Some filtration devices are so effective that they are also capable of removing aerosol droplets such as are significant components of tobacco smoke. However, some air contaminants are gaseous in nature and are not easily removed by filters that rely on interception or entrapment of particulates carried by the air. This problem is particularly acute in closed environments such as airplanes, where the recycling of air containing offensive odors is not calculated to produce contented passengers. It is also of vital concern where it is desired to intercept truly noxious gases, as in chemical warfare situations or in areas where poisonous gases have been accidentally vented. On a less critical level, it is often desirable to reduce the moisture or carbon dioxide content of an air flow.
It is known that some materials are highly effective at removing undesirable, (defined herein to include toxic, noxious, offensive, or otherwise unwanted), gases and it has been proposed that recycled filtered air also be passed through a pad of one of these materials before completing the recycle operation. Problems, however, occur when the sorbent material is saturated and gas "breakthrough" occurs. From a flow free from the undesirable gas, the change to an undesirable level of contamination is swift and dramatic because the air flow is, by design, equal through all parts of the pad so far as can be arranged. Thus, breakthrough occurs at all points essentially simultaneously. Since it is not easy to predict or measure the approach of breakthrough, it is necessary to have excessively frequent pad changes or risk breakthrough occurring at an inconvenient or dangerous time.
Another problem with the use of sorbent beds of the prior art is that they usually result in quite a severe pressure drop across the pad. Thus, it is necessary to provide more powerful air circulation fans which, beside needing more power, tend to be noisier and often more bulky. These are serious issues when the system is used, for example, in an aircraft cabin air recycling system.